Fix #20306: Add new yii\helpers\ArrayHelper::flatten() method

framework/helpers/BaseArrayHelper.php

@@ -1043,4 +1043,61 @@ class BaseArrayHelper
 
         return $array;
     }
+
+    /**
+     * Flattens a multidimensional array into a one-dimensional array.
+     *
+     * This method recursively traverses the input array and concatenates the keys
+     * in a dot format to form a new key in the resulting array.
+     *
+     * Example:
+     *
+     * ```php
+     * $array = [
+     *      'A' => [1, 2],
+     *      'B' => [
+     *          'C' => 1,
+     *          'D' => 2,
+     *      ],
+     *      'E' => 1,
+     *  ];
+     * $result = \yii\helpers\ArrayHelper::flatten($array);
+     * // $result will be:
+     * // [
+     * //     'A.0' => 1
+     * //     'A.1' => 2
+     * //     'B.C' => 1
+     * //     'B.D' => 2
+     * //     'E' => 1
+     * // ]
+     * ```
+     *
+     * @param array $array the input array to be flattened in terms of name-value pairs.
+     * @param string $separator the separator to use between keys. Defaults to '.'.
+     *
+     * @return array the flattened array.
+     * @throws InvalidArgumentException if `$array` is neither traversable nor an array.
+     */
+    public static function flatten($array, $separator = '.'): array
+    {
+        if (!static::isTraversable($array)) {
+            throw new InvalidArgumentException('Argument $array must be an array or implement Traversable');
+        }
+
+        $result = [];
+
+        foreach ($array as $key => $value) {
+            $newKey = $key;
+            if (is_array($value)) {
+                $flattenedArray = self::flatten($value, $separator);
+                foreach ($flattenedArray as $subKey => $subValue) {
+                    $result[$newKey . $separator . $subKey] = $subValue;
+                }
+            } else {
+                $result[$newKey] = $value;
+            }
+        }
+
+        return $result;
+    }
 }